1. Field of the Invention
The present invention relates to an ultrasonic probe and an ultrasonic diagnostic apparatus.
2. Description of the Related Art
There is known an ultrasonic diagnostic apparatus that scans the inside of a subject with an ultrasonic wave and visualizes an inside state of the subject on the basis of a reception signal generated from a reflected wave from the inside of the subject. Such an ultrasonic diagnostic apparatus transmits an ultrasonic wave into the subject with an ultrasonic probe including piezoelectric oscillators and receives a reflected wave, which is caused by mismatching of acoustic impedances in the subject, with the ultrasonic probe to generate a reception signal.
In the ultrasonic probe, plural piezoelectric oscillators, which oscillate on the basis of a transmission signal to generate an ultrasonic wave and receives a reflected wave to generate a reception signal, are arranged in a scanning direction. For example, such piezoelectric oscillators transmit an ultrasonic wave having a rectangular sound pressure distribution, which is uniform in a direction perpendicular to the scanning direction, and form a focus at a predetermined depth in a subject when the piezoelectric oscillators are given a differential delay by an acoustic lens.
Incidentally, for the purpose of realizing acoustic matching of an acoustic impedance of the piezoelectric oscillators and an acoustic impedance of the subject, an acoustic matching layer having a multilayer structure is provided on the piezoelectric oscillators to transmit and receive ultrasonic waves via the acoustic matching layer. As the acoustic matching layer, an acoustic matching layer consisting of two layers has more satisfactory acoustic matching than an acoustic matching layer consisting of one layer. An acoustic matching layer consisting of three layers has still more satisfactory acoustic matching. This is because an acoustic loss is less when acoustic impedances change from the piezoelectric oscillators to the subject in three stages than in one stage.
The acoustic matching between the piezoelectric oscillators and the subject is made satisfactory in this way. This is because, if a difference between the acoustic impedance of the piezoelectric oscillators and the acoustic impedance of the subject is large, a reflection loss of an ultrasonic wave in the subject increases when the ultrasonic wave is transmitted from the piezoelectric oscillators to the subject. Consequently, the transmission of the ultrasonic wave to the subject cannot be performed efficiently, and a high quality image cannot be obtained.
FIG. 9 shows a structure of an ultrasonic probe including an acoustic matching layer having a multilayer structure. FIG. 9 is a front view of the ultrasonic probe. The ultrasonic probe includes a back material 32, a piezoelectric oscillator layer 33 that is divided into plural layers to be arranged in a scanning direction on the back material 32, an acoustic matching layer 34 that is divided into plural layers to be arranged in the scanning direction on the piezoelectric oscillator layer 33, and an acoustic lens 35 provided on the acoustic matching layer 34. The acoustic matching layer 34 includes a first acoustic matching layer 34a, a second acoustic matching layer 34b provided on the first acoustic matching layer 34a, and a third acoustic matching layer 34c provided on the second acoustic matching layer 34b. In such an ultrasonic probe, the piezoelectric oscillator layer 33 performs transmission and reception of ultrasonic waves via the acoustic matching layer 34.
In general, an acoustic impedance of the piezoelectric oscillator layer 33 is about 30 Mrayl and an acoustic impedance of a subject is about 1.5 Mrayl. In order to make acoustic matching between the piezoelectric oscillator layer 33 and the subject, it is necessary to form the acoustic matching layer 4 in a multilayer structure and gradually reduce acoustic impedances from the piezoelectric oscillator layer 33 to the subject. In the case of the ultrasonic probe shown in FIG. 9, it is necessary to gradually reduce acoustic impedances from the first acoustic matching layer 34a to the third acoustic matching layer 34c to set an acoustic impedance of an acoustic matching layer on the subject side (the third acoustic matching layer 34c) of the acoustic matching layer 34 to 1.5 to 3.5 Mrayl. In addition, in the case of an ultrasonic probe including an acoustic matching layer consisting of two layers, it is necessary to set an acoustic impedance of the second acoustic matching layer to 1.5 to 3.5 Mrayl.
Conventionally, an acoustic impedance is set low by using a soft resin film of polyurethane or polyethylene in the acoustic matching layer 34. However, since the resin film is poor in machinability due to its flexibility, it is impossible to subject the acoustic matching layer 34 to machining by dice cutting (array machining) in order to divide the acoustic matching layer 34 into plural layers to be arranged in the scanning direction. In other words, after stacking the piezoelectric oscillator layer 33 and the acoustic matching layer 34 on the back material 2, it is impossible to subject the acoustic matching layer 34 to dice cutting at a desired pitch. Therefore, there is a problem in that acoustic crosstalk between the piezoelectric oscillator layer 33 and the acoustic matching layer 34 is high. In addition, since machinability is poor, it is impossible to manufacture the ultrasonic probe easily.
In addition, since polyurethane and polyethylene do not have electric conductivity, it is impossible to draw out a ground electrode from the acoustic matching layer 34 side. Here, even if conductive particles such as a metal filler is mixed in polyurethane or polyethylene in order to give electric conductivity to the acoustic matching layer 34, a desired acoustic impedance is not satisfied because a density of the acoustic matching layer 34 increases.